1. Field of the Invention
The present invention relates to a solid electrolytic capacitor comprising a capacitor element including an anode in which an oxide film is formed on a surface of a valve metal, a cathode furnished with a valve metal, and a separator provided between the anode and the cathode, the anode and the cathode and the separator being wound around together, wherein a solid electrolyte is interposed between the anode and the cathode, an anode lead tab having an oxide film formed on a surface thereof is fixed to the anode, and a cathode lead tab is fixed to the cathode. The invention also relates to a method of manufacturing the solid electrolytic capacitor.
2. Description of Related Art
Electrolytic capacitors that make use of valve metals such as tantalum and aluminum are widely used since they are capable of attaining large capacity with small sized. Among the electrolytic capacitors, solid electrolytic capacitors employing conductive polymers such as polypyrroles, polythiophenes, and polyanilines, or TCNQ complex salts as their electrolytes have attracted attention.
A solid electrolytic capacitor of the foregoing type is fabricated in the following manner (see Japanese Published Unexamined Patent Application No. 6-310381).
First, an anode lead tab that has been subjected to a formation process and a cathode lead tab are fixed to an anode that is made of aluminum foil and has been subjected to an etching process and a formation process, and a cathode, respectively. Thereafter, the anode and the cathode are wound around in a cylindrical form with a separator paper interposed therebetween to form a capacitor element. Further, cut edge formation and a heat treatment is carried out for the capacitor element. Next, the capacitor element is immersed in a solution containing an oxidizing agent and a monomer, and thereafter, thermal polymerization is effected, whereby a conductive polymer layer (a solid electrolyte layer) is formed between the anode and the cathode of the capacitor element. Subsequently, the capacitor element is accommodated and secured in an aluminum case, then the aluminum case is sealed, and further an aging process is performed. Lastly, a base plate is inserted to come into contact with the curled surface of the capacitor, and the lead lines of the capacitor are press-worked and bent to form electrode terminals, so that a solid electrolytic capacitor is completed.
Demands for high-withstanding voltage products of the foregoing solid electrolytic capacitors have been escalating in the market. The solid electrolytic capacitors, however, have a problem that they have poorer self-repairing ability against leakage current (LC) than liquid electrolyte-type electrolytic capacitors. To enable such solid electrolytic capacitors to have an increased withstanding voltage, it is necessary that the withstanding voltage of the dielectric oxide film of the anode foil be high (in other words, the formation voltage for the anode foil needs to be high). Moreover, in the manufacturing process, edges of the anode foil need to be subjected to a formation process again for the same reason. It should be noted, however, that the formation of the edges is a typical process for solid electrolytic capacitors and is not a process that is particularly carried out to increase the withstanding voltage.
Normally, cut edge formation, which is carried out during the manufacturing process, is performed with the same formation voltage as the formation voltage for the anode foil. However, if the formation voltage for the cut edge formation exceeds 200 V (especially over 230 V), the cut edge formation becomes unstable when using a conventional anode lead tab (formation voltage: about 160 V), producing corrosion at many locations in the weld part of the anode lead tab. As a result, the LC characteristics of the solid electrolytic capacitor become unstable.